Flat steel products of the type in question here are typically rolled products such as steel strips or sheets, and blanks and plates produced therefrom.
High-strength flat steel products are growing in significance particularly in the field of motor vehicle construction, since they enable a reduction in the vehicle's intrinsic weight and an increase in the load capacity. A low weight not only contributes to optimal utilization of the technical performance capacity of the respective drive unit, but also promotes resource efficiency, optimization of costs and climate protection.
A crucial reduction in the intrinsic weight of steel sheet constructions can be achieved by an enhancement of the mechanical properties, especially of the strength of the flat steel product being processed in each case. As well as a high strength, modern flat steel products intended for motor vehicle construction are also expected to have good toughness properties, good brittleness resistance characteristics and optimal suitability for cold forming and welding.
It is known that this combination of properties can be achieved by choice of a suitable alloy concept and a specific production method. In the case of conventional methods of producing high-strength heavy plate having a minimum yield strength of 700 MPa, the procedure is as follows. First of all, the slabs are hot-rolled and, after rolling, cooled down under air. Thereafter, the sheets are reheated, hardened and subjected to a tempering treatment. The process thus contains several stages in order to attain the mechanical properties. The multitude of associated production steps leads to comparably high production costs. Exact process control is also required in order to attain the desired toughness properties and surface qualities.
EP 2 130 938 Al discloses a method of producing a hot-rolled flat steel product, in which a melt is cast to slabs containing, as well as iron and unavoidable impurities (in ° A by weight) 0.01%-0.1% by weight of C, 0.01%-0.1% by weight of Si, 0.1%-3% by weight of Mn, not more than 0.1% by weight of P, not more than 0.03% by weight of S, 0.001%-1% by weight of Al, not more than 0.01% by weight of N, 0.005%-0.08% by weight of Nb and 0.001% to 0.2% by weight of Ti, where the following condition applies to the respective Nb content % Nb and the respective C content % C: % Nb×% C≤4.34×10−3.
After the casting and solidification of the melt, in the known method, the steel slab is reheated up to a temperature range having a lower limit which is determined as a function of the C and Nb contents of the steel being cast in each case and an upper limit of 1170° C. Subsequently, the reheated slab is rough-rolled at an end temperature of 1080-1150° C. After waiting for 30-150 seconds, in the course of which the reheated slab is kept at 1000-1080° C., the preheated slab is then hot finish-rolled to give a hot strip. The forming level in the last draft of the hot rolling should be 3%-15%.
In the known process, the hot rolling is ended at a hot rolling end temperature corresponding at least to the Ar3 temperature of the steel being processed and of not more than 950° C. After the end of the hot rolling, the hot strip obtained is cooled down at a cooling rate of more than 15° C./s to a coiling temperature of 450-550° C., at which it is coiled to a coil.
In the hot strip thus produced, the grain boundary density of the carbon present in solid solution is to be 1-4.5 atoms/nm2 and the size of the cementite grains separated out at the particle boundaries not more than 1 μm. The flat steel products having these properties and having been produced by the known method, given sufficiently high-dose alloy contents, are to have tensile strengths of more than 780 MPa and yield strengths of up to 726 MPa. In this way, the hot strip produced in the known manner is to have a combination of properties of particular suitability for use in automobile construction. Optimal surface characteristics are to be attained by restricting the reheating temperature to which the slab is heated prior to hot rolling to the abovementioned temperature range and hence avoiding excessive formation of scale which would be incorporated into the hot strip surface in the course of hot rolling.